The composite materials offer a high electrical resistance compared to metallic materials traditionally used for fabrication of fuel tanks. While a metallic material can be considered as an isotropic material from the electrical point of view, the subject composite material (e.g. carbon fibre with epoxy matrix) does not have the same behaviour. The low conductivity existing to composite material is due to the fact that the electrical continuity in this material is only caused depending on the fibre direction. The carbon fibres have a certain electrical continuity so that the fibres of this material show high electrical resistance along the direction perpendicular to them (anisotropy) depending on the epoxy matrix.
The behaviour of this material is found reflected in the corresponding scheme of FIG. 1. This scheme shows the two fundamental items of any structure constructed from a composite material ply lay-up. The composite material fibres allow the electrical current to pass through them easily, while the resin that joins and gives consistency to these fibres does not permit the electrical current go through. The resin material becomes degraded when it is subjected to high current densities. The mentioned degradation causes a failure of the mechanical properties as well as lack of leak tightness to those structures containing liquids as in fact it happens with fuel tanks.
Below noted are the phenomena in connection with both moderated or severe electrical discharges (lightning strike) that must be prevented to a structure made of composite material in order to assure the material structural integrity/tightness:                Hot spots. High density of electrical current to certain punctual spots of the structure such us joints or intersection items that may cause high temperature spots. Should this temperature exceed 200° C. (self-ignition point as considered by fuel authorities FAA/JAA), the fuel can reach the flammable point if the estequiometric concentrations adequate to the interior of the fuel tank are conferred.        Structural damage (holes, delaminations, melting of the material . . . ): Structural damage is caused when a structure is subjected to a strong lightning strike. Should this damage be very severe, it can cause a global structural failure.        Sparking phenomena: the electrical current flow passing through materials having different resistivities and being at geometrically separated places may produce electrical difference potential among them. Electrical arc discharges will be caused (sparking condition) thus provoking the ignition of the fuel/inflammable liquid contained to the interior of the structure.        
As previously described, the usage of low electrical conductivity composite materials for the fabrication of fuel tanks generates additional risk to that related to structural damage. It may involve the global/catastrophic failure. This is the reason why the concerned item must be provided with the adequate protection in order to prevent the risk from happening as a result from a severe lightning strike caused by nature (flashes). The system of this invention shows the possibility to provide the fuel tank structure with materials, a number of items and their adequate construction to optimise protection.
The structures made of composite materials and specially carbon fibre materials exhibit an ohmic resistance value of 1000 times above that of one structure equivalent to aluminium material. Upon the experience gained by electrical discharging tests and studies, this circumstance permits to foresee the behaviour of a structure made of carbon fibre (or any other low electrical conductivity composite material). The damage caused is major and of different nature. Therefore, the protection system must be assumed having in mind these properties inherent to the material. The mechanism of damage thus provoked by said electrical discharging onto the carbon fibre structures, when compared to the traditional metallic components (aluminium material), can be distinguished in the following way. While, owing to high temperature existing and the slight distortion failure inherent to the process, the traditional metallic component damage is often shown as a perforation caused by the heating of the material up to reaching its melting state, the carbon fibre composite material shows delamination damage (such as material fibres torn out and ruptured). This damage can be associated with a surface visibly deteriorated. Deterioration of surface proves to be greater than that observed in metallic material showing frequent perforations/holes at the area struck by the electric discharge. The failed electrical properties are no longer limited to a perforation failure with the corresponding local stress accumulated to this area but a degradation of the greater extension properties also appears by the rupture of the material fibres which are mainly responding to support the loads provisioning for the design of the structure.
Considering the high electrical resistance of the composite material structures examined hereto, the traditional hypothesis, used for metallic materials permitting to simplify the electric current pulse that emulates the lightning strike effect to a null frequency, is not applicable to this last type of composite material structures. This makes that the frequency must be considered by involving electrical current peaks, phase displacements and transient currents that increment the effect induced by the same structure and extend the damage caused to the structure. This wants to emphasize other phenomena such as the sparking. This phenomena shows that the potential difference by induced current and phase displacements, can provoke the arc sparking inside the fuel tank.
The invention herein indicated deals with the presentation of a system based on the main guideposts for fabrication of the composite material tanks with the possibility to add metallic items (high conductivity metallic substructures, meshes) to assure for the integrity of the fabricated fuel tank and correct events like the lightning strike.
The invention deals with the practice to construct the concerned item (fuel or other flammable liquid tank) using the optimum arrangement of the exposed items to assure that when a lightning strike occurs, the fuel tank has the capability to conduct the electrical current by building up part of the electric arc. Also, the way how the airplane outer skin, internal structural parts, fasteners (rivets), metallic meshes and other items specifically devoted to protection are arranged, permits to achieve a fuel tank capable to support the direct effect of an electric current discharge. Those consequences coming from the lightning strike will be remedied and the derived effects of this event helped too.